CA1244299A

CA1244299A - Spraying apparatus

Info

Publication number: CA1244299A
Application number: CA000502220A
Authority: CA
Inventors: Timothy J. Noakes
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Battelle Memorial Institute Inc
Priority date: 1985-02-19
Filing date: 1986-02-19
Publication date: 1988-11-08
Also published as: EP0194074B1; ZW3886A1; PL258017A1; ZA861004B; FI84026C; EG17766A; NZ215181A; PL157213B1; US4801086A; SU1528331A3; FI860725A0; JPS61227863A; HUT40933A; ES8700971A1; IL77898A; DK77786A; ES552176A0; HU208092B; ATE110594T1; DE3650046T2

Abstract

An apparatus and process for the electrostatic spraying of a mixture of a plurality of liquids, suitably liquids which react together rapidly to form a solid, liquids which are physically incompatible, or liquids, such as paints, to provide novel optical effects. The apparatus includes a sprayhead formed with a plurality of channels which communicate with a common outlet means. The liquids are supplied to respective channels and meet at the outlet means.
where they are subjected to an electrical field which causes a mixture of the liquids to be drawn from the sprayhead in the form of one or more filaments, the or each filament containing a mixture of liquids in the proportions equal or substantially equal to the proportions in which the liquids were supplied to the sprayhead.

Description

~4'~ PP 33375Z

SPRAYING APPARATUS

This in~ention relates to electrostatic spreying.
It is sometimes necessary to coat articles with a material formed from a mixture of liquids which react together rapidly to form - a solid, or to make articles in a particular form, for example beads or filaments, from a mixture of such liquids. There are other proce~ses ~lich involve subjecting a liquid to some treatment for i which it9 phy~ical properties are unsuitable, in which case the liquid may be mixed with a carrier liquid of suitable properties. On other i ooea~ion~ liquids mu3t be mlxed and proees~ed in a manner whieh eau~e9 undesirable changes in the properties of one of the liquid~. Finally, it i8 sometimes desirable to mix differently eoloured liq~ids, sueh as paints, in order to produce novel optieal effeets on a target~
In each of these cases there i9 a need for an apparatus wherein mixing of the liquids is delayed for as long as po~ible before the final proeessing of the mixture takes plaee.
Aeeording to the invention there i9 provided an apparatus for the eleetrostatie spraying of a plurality of liquid~, the apparatus eomprising a ~prayhead formed with a plurality of ehannels ~j each eommunicating with an outlet means at which liquids flowing through respeetive channel3 meet, and means for subjeeting liquids emerging from the outlet means to an electrieal field suffleiently ¦ high for a mixture of liquids to be drawn from the sprayhead in the form of at least one filament, the or each filament containing a 3~

mixture of liquids in proportion~ equal or sl~bstantially equal to the proportions in which they were supplied to the apparatus.
The spra~yhead may comprise ~ serie.~ of mutually spaced plates, each channel being formed by the space between a pair of adjacent plates.
In this case, the sprayhead may comprise a central plate and two outer plates, a channel is formed between each outer plate and the central plate, and the outlet means comprises an outlet edge of each of the plates, the outlet edge of the central plate being located downstream of the outlet edges of re~pective outer plates.
Suitably, the angle which i~ included b~tween opposed ~ides o~ the central plate at the outlet edge thereof i9 smaller thAn the included angle between outer ~ides of respective outer plates.
Preferably, the said angle between oppo~ed sides of the central plate is between 10 and 60, and the angle between outer sides of re~pective outer plates is b~tween 80 and 150.
Alternatively, the ~prayhead may comprise a series of coaxially arranged, generally tubular element~, and each channel i9 formed by a space of generally annular section between two adjacent elements-The ~prayhead may then comprise radially inner, intermediate and outer guide elements, and the outlet means compri~es axially outer edges of respective elements, the axially outer edge of the intermediate element being located downstream of the axially outer edges of the inner and outer elements.

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124~99 Suitably, the angle which, in an axial section of the ~prayhead, ig included between opposed side~ of the intermediate element ~t the ~xiall~ outer ed~e thereof is 3maller th~n the angle between a radially outer side of the outer element and a radially inner side of the inner element.
Preferably, the said angle between opposed sides of the intermediate element is between 10 and 60, and the said angle between the radially outer side of the outer element and the radially inner side of the inner element is between 80 and 150.
Suitably, the outlet means comprises a surface of conducting or semi-conducting material, and the means for subjecting the liquids to an electrical field comprise means for applying an electrical potential to the said surface. Alternati~ely, the outlet means may be formed of non-conducting material and an electrode may be arranged a short distance upstream of the outlet means and at a location such that the electrode is contacted, in use, by at least one of the liquids, the means for subjecting the liquids to an electrical field comprising means for applying an electrical potential to the said electrode.
Preferably, an electrode is mounted adjacent the sprsyhead, and the means for ~ubjecting liquids emerBin~ from the outlet means to an electric field compri9e means for causing a first potentisl to be applied to the liquids, and means for maintainine the electrode at a second potential, the difference between the first and second potentials being 9ufficient to cause formation of the said filament or filaments.

~2~

When ~praying a target at zero potential, the first potential may be 1 to 20KV and the 3econd potential may be at or near earth potential, as disclosed in our UK specification No~ 1.569.707.
Alternatively, when spraying a target at zero potential, the first potential is 25 to 50KV, and the second potential is 10 to 40KY, ; as disclosed in our co-pending Canadian Application No. 498,354, - Preferably, the electrode comprises a core of conducting or semi-conducting material sheathed in a material of dielectric strength and volume re~istivity sufficiently high to prevent sparking between the electrode and the sprayhead and volume rasistivity sufficiently low to nllow char~e collectod on tho sur-~ace of the sheathing material to be conducted throueh tha-t material to the conducting or semi-conducting core. Suitably, the volume resistivity of the sheathing material is between 5 ~ 10 and 5 x 10 3 ohm cms., the dielectric strength of the sheathing material is greater than 15KV/mm and thicknes3 of the sheathing material i8 0.75 to 5.00 mms., preferably 1.5 to 3 mms. Sheathed electrodes of this form are also disclosed in our co-pending Canadian Application No. 498,354.
Means may be provided for supplying the plurality of liquids to the sprayhead 90 that the or each filament becomes unstable and breaks-up into charged droplets a short distance away from the outlet means.
In this case, means may be pro~ided for causing a stream of gas to flow through the region of the high electrical field, the direction and velocity of the stream of gas being such that charged droplets of liquid are removed from the said region, theraby to reduce ~ .
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a build-up in space charge which affects the magnitude of the electrical field. ~he velocity of the stream of gas may be approximately equal to or greater than the velocity of the droplets in the absence of the stream of gas. Spraying apparatu~ in which such a stream of ga~ is employed is disclosed in our co-pending Canadian Application No. 502,219.
Alternatively, means may be provided for supplying the plurality of liquids to the sprayhead so that the mixture of liquids remains in the form o~ a filament or filaments until striking a target.
In apparatus where a stream of gas i9 provided the target and the above-mentioned flrs-t potential may both be at earth potential and the second potential above 5KV.
According to the invention there i8 also provided a process for the electrostatic ~praying of a plurality of liquids comprising supplying the liquids to respective channels in a sprayhead, each channel communicating with an outlet means at which the liquid~
flowing`through respective channels meet, and subjecting liquids emereing from the outlat means to an electrical fiald sufficiently high for a mixture of liquids to be drawn from the ~prayhead in the ~orm of at least onè filament, the or each filament containing a mixture of liquid~ in proportion~ equal to the proportions in which they were 3upplied.
The invention will now be de~cribed, by way of example, with reference to the accompanying drawings, in which:-12~

Figure 1 is a diagrAmmatic side elevation of a sprayhead in a first electrostatic spraying apparatus according to the invention;
Figure 2 is a section on the line I-I of Figure 1;
Figure 3 is an enlarged side elevation of a part of the sprayhead of Figures 1 and 2; anA
Figures 4 to 1~ are ~chematic drawin~s of sprayheads in further electrostatic Ypraying apparatus according to the invention.
The ~prayhead shown in Figure~ 1 to 3 of the drawings is suitable for spraying two liquids.
Referring to Figure~ 1 to ~, the pre9ent sprayhead includes three mutually spaced, parallel arranged plate~, a ccntrfll plate 1 and two outer platas ~ and 5. A ~uppl~ channol for li~uid i~ formod by the space between each pair of adjacent plates. Thus, the space between the plates 1 and 3 forms a first channel 4, associated with which is a distribution gallery 8 and an inlet pipe 13. A second channel 6 i3 formed by the ~pace between the plates 1 and 5 and has an associated gallery 9 and inlet pipe 15~ Each of the channels 4 and 6 i9 approximataly 150 pm wide. As shown in Figure 2, a lower outlet edge 7 of the central plate 1 i~ sharp and is located a short distance below or dow~stream of the lower outlet edges 10 and 12 of re~pective outer plates 3 and 5. The region containing the lower edges 10 and 12 o f the outer plates 3 and 5 and the lower edge 7 o f the central plate 1 ~erves as fln outlet me~ns for thé sprayhead.
Each of the plates 1, 2 anA 3 is made of conductine or semi-conducting material, including the gurfaces of these plates in the ~ ~ ou~let mean~. The plates are connected to an output terminal of a : : :
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volta~e ~enerator (not ~hown)which provides an output voltage of approximately 40KV.
In use, an ~rticle 16 which is to be coated i~ maintained at earth potential and i~ dieposed approximately 5 cms below the S sprayhead, as shown in Figures 1 and 2. The ~enerator is switched on, liquid from a first supply tank is supplied to the sprayhead via the inlet pipe 13, an~ liquid from a second ~upply tan~ is supplied to the ~prayhead via the inlet pipe 15.
A liquid A from the inlet pipe 13 flows into the gallery 8 and then downwardly through the channel 4 whilst a liquid B from the inle-t pipe 15 flown into the gallcry ~ ~nd downwnrdly via the ahannel 6. Upon reaching the outlet means o-f the 3prayhead, the liquid A from ~ the channel 4 moves past the lower outlet edee lO of the outer plate 3 '. and then flow3 downwardly across one face of the central plate 1.
, 15 Liquid B from the channel 6 moves pa~t the lower outlet edge 12 of the ,~ outer plate 5 and then flown downwardly acros~ an opposite face of the central plate 1. The liquid~ A and B mix together once they reach the lower outlet edge 7 of the central plate 1.
~, T]~e potential which is applied to the plates 1, 3 and 5 from , 20 the generator produces an electrostatic field of high i.ntensit~y , ~approximately 8KV/cm) between the lower edge 7 of the central plate 1. The effect of this field is to draw the liquids A and B emerging from the edge 7 into a series of mutually spaced filaments 20, as shown in Figure 1 of the drawings. The spacing between adjacent filaments 20 is determined by the magnitude of the electrostatic field, the properties of the liquids, and the flow rates. Mixing ?

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occurs because all of the liquid~ from the channels 4 and 6 which flows downwardly between the line3 G-G and H-H of Figure ~ is drawn into the filament 20 between thos~ two line~.
As shown in Figure 3, the mixed liquids A and B in each filament 20 subsequently break-up into droplets 21 due to the instability of the liquid jet in air.
The sprayhead of Figure 4 correspond~ to the sprayhead of Figure 2 in that there i~ again a central plate l and two outer plates ~ and 5, respectively, which define supply channels 4 and 6 for re~pective first and ~econd liquids. As ~hown, an outlet edge 7 of the central plate 1 is sharp and is located a short distance below or downstream of the outlet edges 10 and 12 of respective plates 3 and The pre~ent ~prayhead differs from the sprayhead of Figure 2 in that two mutually spaced, parallel arranged electrode elements aredispo~ed adjacent to the outlet edge 7 of the central plate 1. Each of the electrode elements 9 extends parallel to the edge 7 and each electrode element is supported by an insulating arm 11. Each element 9 hss a cors of conducting or semi-conducting material sheathed in a material of dlelectric strength greater than l5KV/mm., volume resistivity between 5 x lO1l and 5 x lO1~ ohm. cms., and thickness 0.75 to 5 mms. This is sufficient to prevent sparking betwe0n the electrode element~ and the ~prayhead. On the other hand, the volume resistivity i~ sufficiently low to allow charge collected on the surfsoe of the ~heathing materisl to be conducted through that .

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material to the core. The 3pecific resistance of the sheathing material is between 5 x 10 and 5 x 10 2.
There i~ a spacing of 5 to 10 mms between each electrode element 9 and the outlet edge 7 and the two elements 9 are ~paced apart by approximately 8 to 20 mms.
In use, a target i9 again maintained at earth potential, the plates 1, 3 and 5 are maintained at an electrical potential of 25 to 50KV, and the electrode elements 9 are maintained at a potential of 10 to 40KV. Alternatively, the plates 1, 3 and 5 can be maintained at 1 to 20KV and the element~ 9 at or near earth potential.
As with the sprayhead of Figure 2, the liquids from the channels 4 and 6 flow downwardly on re~pective opposite faces of the plate 1 before meetinB at the edge 7, where they mix. The presence of i the electrodes 9 serve~ to intensify the electrostatic field at the lS edge 7, and hence to improve atomisation of the mixture of liquids emerging from that edge.
Figure 5 of the drawing3 shows a side elevation of a ~prayhead in a further apparatus according to the invention. The sprayhead of Figure 5 corre~ponds to the sprayhead of Figure 2 except that a central plate 25 of the sprayhead has an outlet edge 26 which is toothed rather than straight. As shown in Figure 5, one filament 27 i8 now formed at 0ach tooth, unless the teeth are too alose together, when some teeth will not have filaments, or too far apart, ~i when some teeth may have more than one filament.
: :: :
Referring now to Figure 6, a further sprayhead in apparatus ::
i ac~ording to the invention is designed for mixing three liquids. The .
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--1 ()--~,prayhead inclu(les two inner plates 31 and 32 and two outer plates ~3 and 34, which together define three channels 35, 36 and 37 for respective liquids. The inner plates 31 nnd 32 have outlet edges which are sharp and which are located R short distance downstream or below the outlet edges of the outer plates 33 and 34.
In use, the liquid which is supplied to the channel 35 moves past the lower edge of the outer plate 33 and then flows downwardly on one face of the inner plate 31 to the outlet edge of that plate.
Liquid from the channel 37 likewise flows do~1wardly to the outlet edge of the inner plate 32. At the outlet edges of the inner plates 31 and 32 the liquids from channels 35 and 37 meet and mix with the liquid flowing down the channel 36.
Figure 7 of the drawin~ shows a sprflyhead which has an annular outlet moans a~ compared w1th the linear outlet means of the sprayheads of Figures 1 to 6.
~eferring to Figure 7, the sprayhead is formed of radially inner, intermediate and outer elements 41, 43 and 45, respectively, each of which is generally tubular in shape. The elements 41, 43 and 45 are coaxially arranged ~o that a first channel 47 is formed between the elements 41 and 43 and a second channel 49 i9 formed between the elements 43 and 45. The intermediate element 43 i9 arranged with its lower outlet edge a short distance below the outlet edges of the inner elem0nt 41 and the outer element 45.
In the sprayhead of Figure 7, mixing of liquids supplied to the channels 47 and 49, respectively, takes place at the outlet edge ~ of the intermediate element 43, In the anner described above.

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Figure 8 of the Arawings ~hows a further sprayhead in which channel~ 41 and 43 for liquids are defined by upstanding plates 45, 47 and 49 of insulating material. In this case an electrode 51 is formed by a meta~ insert at a lower edge of the plate 47 and an intense S electrostatic field is developed at that lower edge by applying a suitable potential to the electrode.
In the sprayhead of Fi~ure 9 there are again three plates of insulating material defining two channels for liquids. In this case, electrodes 51 and 53, each contacting the liquid in a respective one of the channels, are provided for use in developing an intense electrostatic field at the lower edge of the centr~l plate.
The flpparatus of Figure 9 can be modified by using only one of the electrodes 51 nnd 53.
Figures 10 and 11 show a sprayhead having a body 61 of conducting material which has a generally conical tip and i~ formed with four channels 63, 65, 67 and 69 for liquids. Each of the channels 63 to 69 extends downwardly through the body 61 to an outlet at the tip.
In use. four liquids are supplied to respective channels 6~, 65, 67 and 69 and meet at the tip of the body 61. At the tip the liquids mix and are subjected to an electrostatic field which cau9es them to be drawn into filaments.
Figure 12 is R sprayhead suitable for mixing two liquids A
and B whose physical properties make it Aifficult to obtain thorough mixing. In the apparatus of Figure 12 there are four channels 71, 73, 75 and 77 defined by up~tanding plates 79, 81, ~, 85 Rnd ~7. The ~' plate~ 79 to ~7 are ma~e of insulating ma~erial and an electrode ~9 is therefore provided at a lower outlet edge of the central plate 83.
In use, a first liquid A is supplied to the chann~ls 71 and 75 and a second liquid B is supplied to the channel~ 73 and 77. The liquid~ A qnd B in respective channel~ 71 and 73 meet at a lower outlet edge of the plate 81 and the liquid3 A and B in respective channels 77 and 75 likewise meet at a lower outlet edge of the plate 85. Mixing begins as the liquids then flow down on re3pective opposite sides of the plate 83 and is continued when the two partial mixtures meet at the lower edge of that plate. The liquids are then subjeateA to an Intense electric~l field which effects atomisation.
The ~prayhead of Figure 12 can al90 be used for mixing four different liquids, ~uch as paint3, to produce a desired optical effect on e target. In thi3 case liquid3 A, B, C and D are supplied to respective channels 71, 73, 75 and 77.
Figure 13 i3 a sprayhead according to the invention which i~
also particularly ~uitable for mixing liquids where difficulties are experienced in obtaining thorGugh mixing.
In this connection, it will be appreciated that any two liquids which flow into the outlet mean~ of the sprayheads described above are charged to the same polarity as they move towards the location at which one of the liquids contacts the other. For example, in the sprayhead of Figures 1 to ~ the liquid3 flowing downwardly on re3pective opposite sides of the central plate 1 are charged to the ;~ same polarity a3 they approach the outlet edge 7 of that plate. In ~: .

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the result, there i~ ~ tendency for the liquids to repel one another as they meet at the edge 7. Indeed, in extreme cases the two liquid3 may emerge from the edge 7 as separate streams.
To overcome this problem it is possible to employ plates of insulating material, as in Figure 9 for example, and to provide an electrode in only one of the channels between the plates. One of the liquids is then charged and the other is uncharged. Unfortunately, however, this may result in the charged liquid being deflected sideways as it moves downwardly past the electrode elements adjacent the ~prayhead~
It has now been appreciated that there are two conflicting requirement~ in de~igning a ~prayhead which will overcome this problem.
On the one hand, providing a central plate with a Rharp outlet edge (ie. a small included angle between respective opposite sides of the plate at the outlet ed8e thereof) results in a more intense electrical field in the immediate vicinity of the sprayhead.
This improYes atomisation. On the other hand, the sharpness of the outlet edge has the result that there i3 a wide range of angular directions a'Long which there is a high potential gradient. There is therefore a tendency for liquid emergine from the sprayhead to be dispersed over a wide angle.
Ag~inst this 9 a blunt ou~"Let edge (ie. an outlet edge having a large included angle between respective o,pposite sides of the plate at the outlet edge) re~ults in a le~ intense electrical field but a well directed stream of liquids.

.
' ' Referring now to Figure 13, a further sprayhead according to the invention has a central plate 91 and two outer plates 93 and 95, providing channels 97 and 99. An outlet edge 101 of the central plate 91 is sharp ie. there is an included angle of ~0 between respective oppose~ sides of the plate 91 at the edge 10t. Outlet edges 103 and 105 of respective plates 93 and 95 are disposed 2 to 3 mms. above the edge 101 of the plate 91. There is a blunt included angle of 120 between an outer side of the plate 93 and an outer side of the plate 95 in the region of the outlet means, (ie. in the region where each outer side ~lopes inwardly and downwardly towards an edge 103 or 105).
In u~e of the sprayhead of Figure 13 it i9 found that the sharp edee 101 of the central plate 91 results in an inten~e electrical field sufficient to eivo ~ood ntomL3ntion. On the other hand, the large angle between the outer 3ides of respective plates 93 and 95 produceY an electrical field such that there is a high potential gradient only in a vertically downwards or sub~tantially vertically downwards direction. Liquid3 therefore emerge from the sprayhead in a narrow, well-defined stream.
The sprayhead of Figure 13 may have plates of conducting or 3emi-conducting material or it may have in3ula-ting plates with electrode3 in the form of metal inserts.
A further sprayhead according to the invention has annular outlet mean~, as is the case for tlie sprayhead of Figure 7. In this further sprayhead, however, the intermediate tubular element corre~pondine to the element 43 of Fi~ure 7 ha~ an outlet edge which .

29~

is 2 to ~ mms. below the outlet edges of the radially inner and outer elements. Moreover, when viewed in axial section (as ahown in Figure 7) there is an included angle of 20 between radially inner and outer sides of the intermediaf,e element in the re~ion of the outlet edge. Between a radially outer side of the outer element and a radially inner side of the inner element there is an included angle of 9oo ~
In general, it is found that satisfactory results for the sprayhead of Figure 13 and the corresponding sprayhead with ~nnular outlet means can be obtained with an included angle of 10 to 60 for the sharp edge of the central plate or intermediate element and ; 80 to 150 for the included angle between the relevant sides of , the other plates or elements.
In eaah of the above ~prayheads it is found that an electrical field of 5 to 30KV/cm. is ~ufficiently high to draw liquids from the sprayhead~ in the form of filaments.
Each of the sprayheads show,n in Figures 4 to 13 may be provided with electrode elements, as in the sprayhead of Figure 4. In the case of the sprayhead of Figure 7, ring-shaped electrode elements are provided.
Each of the appAratus described above can be used for mixing a variety of di~ferent liquids.
Fir~t, the apparatu~ ia ~uitable for coating articles with a material formed from a mixture of two liquid components which react ~;~ 25 together rapidly to form a solid. However, the reaction time must be ;~ suf~icient for the or each filament emer6ing from the sprayhead to remain in liquid form until the filament has become unstable and ~ ':

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~2~2~

_15-broken up into charged liqui~ ~roplets. Solidification must then take place after the droplets have landed on an article to be coated.
~iquids which can be used are monomsrs and/or prepolymers with or without catalysts, blowing agents and pigments.
Examples are (1) Polymeric foams such as polyurethane, where the liquid component~ are polyol and di-isocynate, one or each dissolved in a blowin~ agent.
(2) Rapidly curing two-pack paint systems.

(3) Thin polymeric films such a~ ~ilicone coatings, where the liquid components may be 5O% silicone polymer, dissolved in a solvent with 4~ of a platinum catalyst, and 5O% silicone polymer, also dissolvod in ~ ~olvont and with 4~ of a silicone cross-linking polymer.
~,~ 15 (4) Two-pack adhesive systems.
The article or target coated by such materials may be hand-held. In this case, the apparatus is particularly suited for use in coating articles of complex shape. Hard coatings are readily applied.
Alternatively, the article may be a sheet moving along a ¦ production line. A sprayhead having n linearly extending outlet, ¦ transverse to the direction of movement of the 3heet is then particularly suitable.
g Secondly, each of the apoaratus described above can be used ¦ 25 to make articles in t;he form of beads or filamcnts.In the case of bea'ds, the liquid components must react together to form ~ solid after ::

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each liqui~l filament has broken up into ch~rged liquid droplet~ but before the droplets have landed on a target. In the ca~e of filament production, the liqui~ component~ mu~t react together to form a solid filament before each liquid filament from the sprayhead has had time to break up into charged droplets. The resulting solid filament is ' continuously wound on to a support at the rate at which it i~ being produce~. It will be apreciated that liquids having a fast reaction time can be employed.
Thirdly, each apparatus described above can be u~ed to effect atomiRation of phy~ically incompatible liquids. An example ariseA in agricultural and other kinas of spraying, where it may be de~irable to ~pray together a colloid and a liquid which, upon aontact ~ with colloid, would cause it to flooculate. With the above Apparatus, t the colloid does not contact the liquid until they are emerging from the sprayhead. There is then no time for the colloid to be degraded by flocculation.
Finally, each apparatus can be used to spray a liquid whose electrical properties, for example resi9tivity, would otherwise render the liquid unsuitable for electrostatic spraying. In this case, the ~ 20 apparatus i9 supplied wi-th the Apraying liquid and with a carrier ,~ liquid of appropriate resistivity. Such apparatus i~ particularly ~ useful for agricultural spraying.

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Claims

CLAIMS PP.33375Z

1. An apparatus for the electrostatic spraying of a plurality of liquids, the apparatus comprising a sprayhead formed with a plurality of channels each communicating with an outlet means at which liquids flowing through respective channels meet, and means for subjecting liquids emerging from the outlet means to an electrical field sufficiently high for a mixture of liquids to be drawn from the sprayhead in the form of at least one filament, the or each filament containing a mixture of liquids in proportions equal or substantially equal to the proportions in which they were supplied to the apparatus.

2. An apparatus as claimed in claim 1, wherein the sprayhead comprises a series of mutually spaced plates, and each channel is formed by the space between a pair of adjacent plates.

3. An apparatus as claimed in claim 2, wherein the sprayhead comprises a central plate and two outer plates, a channel is formed between each outer plate and the central plate, and the outlet means comprises an outlet edge of each of the plates, the outlet edge of the central plate being located downstream of the outlet edges of respective outer plates.

4. An apparatus as claimed in claim 3, wherein the angle which is included between opposed sides of the central plate at the outlet edge thereof is smaller than the included angle between outer sides of respective outer plates.

5. An apparatus as claimed in claim 4, wherein the said angle between opposed sides of the central plate is between 10° and 60°, and the angle between outer sides of respective outer plates is between 80° and 150°.

6. An apparatus as claimed in claim 1, wherein the sprayhead comprises a series of coaxially arranged, generally tubular elements, and each channel is formed by a space of generally annular section between two adjacent elements.

7. An apparatus as claimed in claim 6, wherein the sprayhead comprises radially inner, intermediate and outer guide elements, and the outlet means comprises axially outer edges of respective elements, the axially outer edge of the intermediate element being located downstream of the axially outer edges of the inner and outer elements.

8. An apparatus as claimed in claim 7, wherein the angle which, in an axial section of the sprayhead, is included between opposed sides of the intermediate element at the axially outer edge thereof is smaller than the angle between a radially outer side of the outer element and a radially inner side of the inner element.

9. An apparatus as claimed in claim 8, wherein the said angle between opposed sides of the intermediate element is between 10° and 60°, and the said angle between the radially outer side of the outer element and the radially inner side of the inner element is between 80° and 150°.

10. An apparatus as claimed in claim 1, wherein the sprayhead comprises a body having a generally conical tip, and each channel extends through the body to an outlet at or near the tip of the body.

11. An apparatus as claimed in claims l, 2 or 3, wherein the outlet means comprise a surface of conducting or semi-conducting material, and the means for subjecting the liquids to an electrical field comprise means for applying an electrical potential to the said surface.

12. An apparatus as claimed in claims 1, 2 or 3, wherein the outlet means is formed of non-conducting material, and an electrode is arranged a short distance upstream of the outlet means and at a location such that the electrode is contacted, in use, by at least one of the liquids, the means for subjecting the liquids to an electrical field comprising means for applying an electrical potential to the said electrode.

13. An apparatus as claimed in Claim 1, wherein an electrode is mounted adjacent the sprayhead, and the means for subjecting liquids emerging from the outlet means to an electric field comprise means for causing a first potential to be applied to the liquids and means for maintaining the electrode at a second potential, the difference between the first and second potentials being sufficient to cause formation of the said filament or filaments.

14. An apparatus as claimed in claim 13, wherein, for spraying a target at zero potential, the first potential is 1 to 20KV and the second potential is at or near earth potential.

15. An apparatus as claimed in claim 13, wherein, for spraying a target at zero potential, the first potential is 25 to 50KV and the second potential is 10 to 40KV.

16. An apparatus as claimed in claim 13, wherein the electrode comprises a core of conducting or semi-conducting material sheathed in a material of dielectric strength and volume resistivity sufficiently high to prevent sparking between the electrode and the sprayhead and of volume resistivity sufficiently low to allow charge collected on the surface of the sheathing material to be conducted through that material to the conducting or semi-conducting core.

17. An apparatus as claimed in claim 16, wherein the volume resistivity of the sheathing material is between 5 x 1011 and 5 x 1013 ohm. cms., the dielectric strength of the sheathing material is greater than 15KV/mm. and the thickness of the sheathing material is 0.75 to 5.0 mms.

18. An apparatus as claimed in claim 16, wherein the thickness of the sheathing material is 1.5 to 3 mms.

19. An apparatus as claimed in claim 16, wherein the specific resistance of the sheathing material is between 5 x 1010 and 5 x 1012.

20. An apparatus as claimed in Claim 1, comprising means for supplying a plurality of liquids to the sprayhead such that the or each filament becomes unstable and breaks up into charged droplets a short distance away from the outlet means.

21. An apparatus as claimed in claim 20, wherein means are provided for causing a stream of gas to flow through the region of the high electrical field, the direction and velocity of the stream of gas being such that charged droplets of liquid are removed from the said region, thereby to reduce a build-up in space charge which affects the magnitude of the electrical field.

22. An apparatus as claimed in claim 21, wherein the velocity of the stream of gas is approximately equal to or greater than the velocity of the droplets in the absence of the stream of gas.

23. An apparatus as claimed in claims 1, 2 or 3, comprising means for supplying a plurality of liquids to the sprayhead such that the mixture of liquids remains in the form of a filament or filaments until striking a target.

24. A process for the electrostatic spraying of a plurality of liquids comprising supplying the liquids to respective channels in a sprayhead, each channel communicating with an outlet means at which the liquids flowing through respective channels meet, and subjecting liquids emerging from the outlet means to an electrical field sufficiently high for a mixture of liquids to be drawn from the sprayhead in the form of at least one filament, the or each filament containing a mixture of liquids in proportions equal to the proportions in which they were supplied.